The present invention relates to a waveguide type optical multiplexer/demultiplexer the wavelength characteristics of which can be finely adjusted by an electric signal.
Multiplexed optical wavelength transmission technology in optical fiber communications is important for economizing a communication system. In the multiplexed optical wavelength transmission, an optical multiplexer/demultiplexer is an indispensable device.
Regarding the construction of the optical multiplexer/demultiplexer, researches have recently been made on a waveguide type structure which is intended for reduction in cost and 1-chip monolithic implementation. An example thereof is a directional coupler type optical demultiplexer (N. Takato et al.: "Low-loss Directional Coupler employing Highsilica Embedded Channel Waveguides," OEC '86, Technical Digest, A3-3, July 1986). As illustrated in FIGS. 9(a) and 9(b), it consists in a method wherein two optical waveguides are arranged in parallel, and optical demultiplexing characteristics are attained by utilizing the wavelength dependence of the coupling between the two optical waveguides.
Since the directional coupler type optical demultiplexer in the prior art has the simple construction, it has the merit of easy fabrication. However, it has the problem that the shift of a center wavelength arises due to the fluctuations of structural parameters during the process of manufacture, and lowering in the available percentage has been inevitable. FIGS. 10(a)-10(c) show the analytical results of the sensitivity of the optical demultiplexer as calculatively obtained by the inventor, and in each of the figures, the wavelength is taken on the axis of abscissas, while the loss is taken on the axis of ordinates. FIG. 10(a) indicates the results in the case where the core width W of each waveguide was caused to deviate .+-.5% from its set value of 10 .mu.m, FIG. 10(b) the results in the case where the core thickness T of each waveguide was caused to deviate .+-.5% from its set value of 8 .mu.m, and FIG. 10(c) the results in the case where the difference .DELTA.n between the refractive indices of the core and cladding of the waveguide was caused to deviate .+-.5% from its set value of 0.25%. As seen from the figures, the deviations of the structural parameters shift the center wavelength relatively. It is understood that, among the aforementioned deviations, the deviation of the core width W is influential on the wavelength shift. Such a wavelength shift incurs degradation in the crosstalk immunity between the channels. With the prior-art optical demultiplexer, the center wavelength shift ascribable to the fluctuation of any structural parameter cannot be remedied, and it has resulted in the lowering of the available percentage because articles with the center wavelength shifts are not usable.